I have been using BFSU for about two months and I have to say I LOVE this curriculum! I am really lucky to be doing it with a friend who has kids who are close in age to mine. Not because this curriculum is difficult to use/present but because I may have been too intimidated to start if I didn't have the motivation of working with someone else. So far we have done 6 lessons and they have been surprisingly easy to present once you have read (and often reread the lesson).
The lessons also don't take all that long. I think the longest we have spent on formal BFSU at any one time is 45 minutes and that's with 5 kids only 2 of whom are formally participating. So that includes a lot of getting the 3 year olds set up with an age appropriate version of the activity or a snack or shoes so they can go out side etc.
The lessons have not required allot of time to prep, however, I do have to really sit and concentrate when I read the up coming lesson. For me it's not the kind of thing I can read/ prep without giving it my full attention. As I mentioned earlier I sometimes need to read it twice to get a full feel for the lesson, this is not because it is poorly written but rather because the lessons are deep and meaningful when it comes to scientific thought and understanding. I wish that my own early science experiences had been like this.